As is known, a differential pair is formed of two identical transistors biased by the same current source or different current sources.
The differential pair is the active portion of a differential amplifier, for example.
The object of the differential pair is to amplify the difference between two input signals of the differential pair.
The output voltages of the amplifier therefore consist in practice of a differential mode voltage portion and a common mode voltage portion.
The differential mode output voltage corresponds to the difference between the input signals amplified by the differential gain and the common mode output voltage corresponds to half the sum of the output signals or in other words to the average of the output signals.
To avoid output saturation problems and to obtain as linear of an amplification as possible, it is preferable for the common mode voltage to remain close to a predefined value.
One parameter representative of the quality of a differential amplifier is the common mode rejection ratio (CMRR), and corresponds to the ratio between the differential mode gain and the common mode gain. The CMRR parameter is maximized when the common mode gain is minimized.
Moreover, the common mode output voltage is subjected to variations depending on variations of other parameters of the circuit.
Any variation in the dimensions of the transistors of the differential pair during its fabrication can cause an imbalance between the branches of the differential amplifier, causing variations in the common mode output voltage.
Other external factors, for example, temperature, can affect the transistors of the differential pair and the remainder of the amplifier circuit differently, causing variations in the common mode output voltage.
Variations in the common mode output voltage interfere with the performance of the differential amplifier, in particular the differential mode gain and the excursion of the output signals.
It is therefore necessary not only to control the value of the common mode output voltage but also to reduce the dependence of the value of that voltage on variations of other parameters of the circuit.
Techniques have therefore been proposed aiming to control the value of the common mode output voltage of these amplifiers.
One known technique includes extracting the common mode output voltage in order then to compare the extracted voltage and a reference voltage, and finally amplifying the signal resulting from this comparison. This amplified error signal is then fed back into the differential pair via its bias components.
This technique provides good control of the common mode voltage, but implementing it uses up silicon die area and increases the current consumption, which is caused by adding additional components dedicated to extracting the common mode voltage, comparing that voltage with a reference voltage, and amplifying the error signal.
A second technique includes extracting information on the common mode input voltage and injecting it appropriately into the circuit in order to compensate for the impact of its variations on the common mode output voltage.
This technique also necessitates the addition of components. Furthermore, its current consumption is high even though it does not have the accuracy of the first technique. Effectively it just compensates for the impact of the variations of the input signal on the common mode output voltage, rather than controlling the common mode output voltage.
The IEEE document “A 0.5 V Bulk-Input fully differential operational transconductance amplifier” (Shouri Chatterjee, Yannis Tsividis and Peter Kinget, Department of Electrical Engineering, Columbia University, New York, USA) describes a technique for feeding the common mode output voltage back into the bias components. This technique is also less accurate than the first technique, as the common mode output voltage is not compared to a reference voltage.